Effects of prostaglandin F2 alpha, 1-methyl-3-isobutylxanthine (MeIX), prolactin and insulin on development of lipoprotein lipase and accumulation of triacylglycerol was studied in 3T3-LI cells as they differentiated in culture from fibroblasts into adipocytes. The findings indicated that lipoprotein lipase activity in differentiating 3T3-LI cells is increased by hormones and other agents that enhance their conversion to adipocytes. Also, development of lipoprotein lipase activity preceeds by nearly one week the development of fatty acid synthesizing enzymes in differentiating preadipocytes. It is concluded that insulin stimulates synthesis of lipoprotein lipase in mature 3T3-LI adipocytes, and this effect is linked to increased metabolism of glucose. Studies in vivo showed that lipoprotein lipase is involved in uptake of cholesterol and triacylglycerol from chylomicrons by both mammary gland and adipose tissue, and in uptake of phospholipid by mammary gland. The findings also indicate that reciprocal changes in lipoprotein lipase activity in mammary gland and adipose tissue, as during lactation, result in diversion of chylomicron lipids from one tissue to the other.